During the semiconductor Shallow Trench Isolation (STI) production process, there are two main ways of Chemical Mechanical Polishing (CMP) for the STI, i.e. Reverse CMP and Direct CMP. Under normal circumstances, Direct CMP will not be used in a case of the STI layer using SiON as an anti-reflection layer, the reason for this is because remaining silicon oxynitride on SiN surface cannot be removed by phosphoric acid (H3PO4) after STI CMP, thus resulting the SiN cannot be wet peeled off.
Currently, in order to precisely control the critical dimension (CD), silicon oxynitride is usually used as the anti-reflection layer. FIGS. 1A to 1D illustrate a flow chart of a conventional Direct CMP STI process. Referring to FIGS. 1A to 1D, in step (a), an oxidation layer (Pad-Ox) S120 is formed on a silicon substrate S110, a silicon nitride layer S130 is formed on a surface of the Pad-Ox S120, a silicon oxynitride layer (anti-reflection layer) S140 is formed on a surface of the silicon nitride layer S130; in step (b), STI-ETCH is performed to etch a depth of the STI, after STI-ETCH, a silicon dioxide (SiO2) layer S150 is deposited using high density plasma chemical vapor deposition (HDP-CVD); in step (c), the silicon dioxide layer S150 on a surface of the anti-reflection layer S140 is polished via Direct CMP until the anti-reflection layer S140 is exposed; in step (d), the silicon nitride layer S130 is removed by reacting of phosphoric acid (H3PO4) with the silicon nitride layer S130, thus forming an active region and an isolation region.
Referring to FIGS. 1A to 1D, when silicon oxynitride on the surface of the STI layer is used as the anti-reflection layer S140, Direct CMP method would no longer be applicable. The reason is that, during Direct CMP, there is a high selectivity (more than 10:1) of the slurry to SiO2/SiON, such that CMP will stop at the surface of the silicon oxynitride. The slurry can react with the surface of the silicon oxynitride, thus forming a layer of relatively dense complex A (referring to step (d)). The complex A will prevent phosphoric acid from reacting with the silicon nitride, resulting in silicon nitride cannot be removed.
FIG. 2A and FIG. 2B are schematic morphology graphs showing silicon oxynitride as the anti-reflection layer after cleaning the silicon nitride. As can be seen from FIG. 2A and FIG. 2B that, due to the blocking of the complex on the surface of the silicon nitride, the silicon nitride on the most region (in region B and C of the figures) cannot be removed.
In order to address this issue, wafer fabrication factory (FAB) usually employs two circumvent ways:
a), using Reverse STI CMP with different slurries to substitute Direct CMP. The defect of this method is that, an additional reverse mask is required to perform exposure, etching, cleaning, therefore the process is relatively complex, and the yield is low;
b), using an organic bottom anti-reflection coating (Organic BARC) layer as the anti-reflection layer to substitute silicon oxynitride on the STI layer. The defect of this method is that, it will be more difficult to control CD during STI-ETCH using organic BARC, thus the mass production process control difficulty is increased.